Novo: nsights into Droplet-Surface Collisions for Aeronautical Applications. Inicial: Modelling of Droplets-Wall Impingement: Jet Fuel and Biofuel Mixtures

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Publications

Does liquid film temperature affects single drop impact dynamics?

Publication . Mendes, André F. S. F.; Vasconcelos, Daniel; Ribeiro, Daniela; Panão, Miguel; Silva, A. R. R.

The effect of liquid film dynamics in the hydrodynamics of an isolated drop impact is a complex phenomenon and not fully understood. Therefore, in this work, an experimental setup built to characterize the impact of an isolated droplet on heated and unheated liquid films consists of a heating element made of an aluminum block with resistances to produce several impact conditions. The parametric studies include the drop impact velocity and size for different fluids to evaluate their properties effect on the phenomena. The results were compared with existing thresholds in the literature to evaluate their validity and applicability range. This comparison allows us to assess if temperature causes the limits of the thresholds to change drastically or if its influence is negligible. Regarding IC engines, thresholds like splashing and bubble encapsulation are significant since they influence the atomization of the mixture and, consequently, the pollutant emissions.

2023-03-26Conference object

Embargoed access

Influence of wetting behavior on the morphology of droplet impacts onto dry-patterned micro-structured surfaces

Publication . Foltyn, Patrick; Ribeiro, Daniela; Silva, André; Lamanna, Grazia; Weigand, Bernhard

The influence of surface roughness, especially regularly patterned micro-structures on the physical outcomes of droplet impacts, is far from fully understood. In order to get a deeper insight into the physics of the impact phenomena, a systematic experimental study of the morphology on regularly patterned micro-structured surfaces has been carried out. The used structures with different dimensions were grooves and pillars with a square cross section. With the help of plasma activation and plasma polymerization processes, the surface wettability was modified independently from the surface structure and material. Two different test fluids were used, namely, distilled water and isopropanol, impacting with various impact energies onto the patterned surface samples. For a better characterization of the impact process, high-speed images from three different perspectives have been acquired synchronously. Due to the transparent surface material, the bottom perspective using a total internal reflection configuration was able to visualize air entrapment inside the surface structure. To the authors' knowledge, such images are not available in the literature, yet. The outcomes have been qualitatively investigated, summarized, and compared. A dependency of the outcomes on the impact energy, the surface wettability, and the structure dimensions could be clearly shown. In general, increasing impact energy will promote the tendency of splashing. However, roughness features cannot only trigger splashing, but can also inhibit it, for example, crown splashing. Moreover, reproducible arrangements of air entrapment inside the structure could be found, which was addressed by the authors as “cookie” and “button” due to their appearance.

2022-12-12Journal article

Open access

Insights on bubble encapsulation after drop impact on thin liquid films

Publication . Ribeiro, Daniela; Panão, Miguel; Barata, Jorge M M; Silva, A. R. R.

The accurate understanding of the phenomenology of drop impact onto dry/ wetted and cold/heated surfaces is increasingly relevant to implement biofuels in civil aviation. The outcome of drop impact depends on the pre-impact conditions and a seldom researched event is the encapsulation of a bubble when this impact occurs on thin liquid films. Therefore, the goal of the experimental work reported is to investigate the mechanism of this bubble encapsulation. Results show that the mechanism leading to a bubble formation has two stages. In the first stage, after the drop impacts a steady liquid film, a prompt splash occurs followed by a crown splash. The uprising sheet propagates in an almost normal direction relative to the liquid film, but its radius at the base continues to expand, eventually leading to the inward collapse of the crown-bounding rim encapsulating air inside the dome. In the second stage, three different phenomenologies of bubble encapsulation can occur. At the top of the closed crown, one jet (phenomenology 1) or two jets are formed (phenomenologies 2 and 3). For phenomenology 2, the upward jet eventually collapses due to gravitational influence, while the downward jet continues to grow until it reaches the liquid film, attaching to it, stretching and detaching from the top at the hemispheric thin sheet, forming a bubble. In phenomenology 3, the upward jet is high enough to allow its breakup and ejection of one large droplet before the collapse of the upward jet. Many secondary droplets fall on the bubble and one of them will eventually break the dome, leading to more secondary atomization. Additionally, the first perturbation imposed on the liquid film by the droplet impact is studied and an empirical correlation is proposed for its propagation velocity. Finally, bubble geometry is investigated.

2023-03-30Journal article

Open access

Experimental Study of a Single Droplet Impinging Upon a Heated Dry Surface Using Jet Fuel and Biofuel Mixtures

Publication . Ribeiro, Daniela; Barata, Jorge M M; Silva, A. R. R.

The aeronautical sector is dependent on fossil fuels which contribute to a considerable amount of pollutant emissions to the atmosphere. In an attempt to reduce these pollutant emissions in a short period of time and without several changes to the aircraft and their engines, this study focuses on the physical phenomena that happen inside the combustion chamber of an internal combustion engine during fuel injection. However, instead of considering only a conventional jet fuel, an alternative fuel is also investigated. The mixture is composed of at least 50% in volume of conventional jet fuel, as demanded by the current legislation, and a biofuel to decrease the consumption of petroleum-based fuels and to reduce pollutant emissions. The main goal of this study is to experimentally observe droplet impact on a heated dry stainless-steel surface for different fuel mixtures, impact energies (Weber number between 174 and 955, and Reynolds number between 1099 and 12365), and wall temperatures (Tw = 20 − 300 ºC) to identify the different heat regimes and the physical differences between the distinct sets of impact conditions. To achieve that purpose an experimental setup was designed and built including the acquisition of high-speed images, a droplet dispensing system, a heating device to accurately control the temperature of the target surface and the illumination of the impact site. To enhance the knowledge about the impacts, three different perspectives of the phenomena were captured. The typical front perspective, a second perspective where the camera has a ≈ 20◦ angle to the impact surface to capture more details of the impact, and finally, at a lower frame rate, an isometric perspective to measure the evaporation time. From these data, the impact regimes were identified and the influence of surface temperature, fluid properties, and impact energy was analyzed.

2022-09-06Conference object

Open access

Measurement of the lamella thickness during droplet impact onto differently wettable smooth surfaces using an extension of the LASER Pattern Shift Method with naturally occurring patterns

Publication . Foltyn, Patrick; Rihm, Lynn Kristin; Ribeiro, Daniela; Silva, André; Weigand, Bernhard

This study shows that the LASER Pattern Shift Method (LPSM) is a powerful measurement technique for film thickness measurements. In this paper, the approach of the LPSM is extended, which is now able to measure the lamella thickness during droplet impacts on smooth surfaces using the naturally occurring air bubble shadows. With the help of this rather new measurement technique, the influence of different experimental parameters on the lamella thickness could be systematically assessed, e.g., the influence of impact velocity, liquid properties, and surface wettability. Upon comparing the obtained results to an analytical correlation in the literature, good agreement could be found for its validity range.

2021-10-27Journal article

Open access